Add ability to specify substep order (#779)

The substep plugin currently shows each substep in the order in which
it appears in the HTML.  This is not always an ideal fit for some
presentation styles, where it would be helpful to specify a different
order (e.g., to add annotations to an image).

This commit allows users to specify a custom order via the
`data-substep-order` attribute.  Substeps without a
`data-substep-order` attribute are revealed last.

This commit also updates the Substep README to document the new
feature.

js/impress.js

@@ -2081,6 +2081,20 @@
             'useAMPM': false
         };
         break;
+    case 'zh-CN':
+    case 'zh-cn':
+        lang = {
+            'noNotes': '<div class="noNotes">当前帧没有备注</div>',
+            'restart': '重新开始',
+            'clickToOpen': '点击以打开演讲者控制界面',
+            'prev': '上一帧',
+            'next': '下一帧',
+            'loading': '加载中',
+            'ready': '就绪',
+            'moving': '移动中',
+            'useAMPM': false
+        };
+        break;
     case 'en': // jshint ignore:line
     default : // jshint ignore:line
         lang = {
@@ -4009,12 +4023,28 @@
 
     var showSubstepIfAny = function( step ) {
         var substeps = step.querySelectorAll( ".substep" );
-        var visible = step.querySelectorAll( ".substep-visible" );
         if ( substeps.length > 0 ) {
-            return showSubstep( substeps, visible );
+            var sorted = sortSubsteps( substeps );
+            var visible = step.querySelectorAll( ".substep-visible" );
+            return showSubstep( sorted, visible );
         }
     };
 
+    var sortSubsteps = function( substepNodeList ) {
+        var substeps = Array.from( substepNodeList );
+        var sorted = substeps
+            .filter( el => el.dataset.substepOrder )
+            .sort( ( a, b ) => {
+                var orderA = a.dataset.substepOrder;
+                var orderB = b.dataset.substepOrder;
+                return parseInt( orderA ) - parseInt( orderB );
+            } )
+            .concat( substeps.filter( el => {
+                return el.dataset.substepOrder === undefined;
+            } ) );
+        return sorted;
+    };
+
     var showSubstep = function( substeps, visible ) {
         if ( visible.length < substeps.length ) {
             for ( var i = 0; i < substeps.length; i++ ) {
@@ -4030,8 +4060,9 @@
     var hideSubstepIfAny = function( step ) {
         var substeps = step.querySelectorAll( ".substep" );
         var visible = step.querySelectorAll( ".substep-visible" );
+        var sorted = sortSubsteps( visible );
         if ( substeps.length > 0 ) {
-            return hideSubstep( visible );
+            return hideSubstep( sorted );
         }
     };